Open Access

Synthesis and Properties of the La1 − x − y Eu y Ca x VO4 (0 ≤ x, y ≤ 0.2) Compounds

  • O. V. Chukova1Email author,
  • S. G. Nedilko1,
  • A. A. Slepets2,
  • S. A. Nedilko2 and
  • T. A. Voitenko2
Nanoscale Research Letters201712:340

https://doi.org/10.1186/s11671-017-2116-7

Received: 29 December 2016

Accepted: 27 April 2017

Published: 8 May 2017

Abstract

The La1 − x Ca x VO4 and La1 − x − y Eu y Ca x VO4 (0 ≤ x, y ≤ 0.2) micro/nanosized powders were prepared by aqueous nitrate–citrate sol–gel synthesis. Phase composition of the sample depends on the x and y values. The La0.9Ca0.1VO4 is crystallized in monoclinic structure up to the x = 0.1. The La0.9Eu0.05Ca0.05VO4 sample was also attributed to the monoclinic structure. Increasing concentration of europium and calcium ions in La1 − x − yEu y Ca x VO4 solid solutions leads to the change of the crystal structure, and subsequently, stabilization of the tetragonal phase takes place.

The obtained samples were characterized by XRD analysis, SEM microscopy, and IR spectroscopy. Luminescence properties of the synthesized powders were studied. Emission of the La1 − x Ca x VO4 samples is weak and consists of wide bands in the 450–800 nm spectral range. The observed bands at 570 and 630 were ascribed to electron transitions in the distorted VO4 3− vanadate groups. Emission of the La1 − x − y Eu y Ca x VO4 samples consists of narrow spectral lines in the 550–730 nm spectral range. The lines are caused by the 5D0 → 7FJ electron transitions in the Eu3+ ions. The Ca2+ ions incorporation increases the intensity of the Eu3+ ions luminescence. Structure of the spectra depends on Ca2+ concentration and excitation wave length. The carried out analysis has revealed that Eu3+ ions form at least two different types of emission centers in the La1 − x − y Eu y Ca x VO4 samples. The assumption is made that type I centers are formed by the Eu3+ ions in their regular positions in the crystal lattice, while the type II centers have complex structure and consist of Eu3+ ions, Ca2+ cations, and oxygen vacancies.

Keywords

Vanadate Sol–gel Eu3+ Luminescence

Background

Rare earth orthovanadates are very interesting class of compounds which have very important applications in various fields involving chemistry and biology, luminescent nanoparticles, and light transformers [14]. Orthovanadates can exhibit unusual magnetic, optical, thermally activated, and X-ray luminescence properties [5]. Nanosized orthovanadates have also attracted considerable research interest as perspective photocatalyst systems for photocatalytic water splitting. The widely used photocatalyst TiO2 is active only under ultraviolet light irradiation [1, 6], while europium (EuVO4) [7] and lanthanum (LaVO4) [8] orthovanadates are of a special attention as the most promising visible-light-driven photocatalysts [9]. These compounds are chemically stable and non-toxic [10, 11]. Their range of biological applications includes fluorescent probes for single-molecule tracking, drug development, protein detection, and fluorescent bio labeling [2, 11, 12].

Actually, some of the abovementioned applications require vanadate materials with improved efficiency of luminescence excitation under light from near UV and violet spectral ranges [4, 1315]. The search for new vanadate compounds for these needs is carried out using variations of two and more cations in their composition including partial iso- and heterovalent substitutions [1619]. It was shown previously that intensities of luminescent emission of the RE activators in orthovanadate compounds can be effectively increased with the A2+ modifying cations (A = Ca, Sr, Ba, Pb, etc.) [4, 16, 2022]. Moreover, properties of compositions with such heterovalent substitutions are strongly dependent on concentration ratios of the A2+ cations. Besides, our recent investigations have shown that increase of intensity of luminescence emission of rare earth orthovanadates can be also achieved with improvement of method of synthesis and morphology of the nanoparticles [14, 23]. Therefore, we expect that expansion of the best of our recent practices for synthesis of the rare earth orthovanadate nanaparticles onto the same compositions with heterovalent substitutions could bring an additional raise of emission intensity of luminescent orthovanadate nanoparticles. At the first step in this direction, we use the Ca2+ modifying impurities as the cheapest reagent.

The aim of this work is to study synthesis procedures, structural features, and morphological and optical characteristics of the nanosized La1 − x Ca x VO4 and La1 − x − y Eu y Ca x VO4 (0 ≤ x, y ≤ 0.2) compounds. Various methods can be applied for synthesis of the orthovanadates nanoparticles, such as solid state [16, 23], hydrothermal [24], solution combustion [17], and sol–gel methods [14, 25]. One of the most promising ways to perform an excellent homogenization for nanoscale sizes of particles, high reactivity of the compounds, and morphology that satisfy enhanced emission intensity is the sol–gel method.

Methods/Experimental

Synthesis

The La1 − x Ca x VO4 and La1 − x − y Eu y Ca x VO4 samples (0 ≤ x, y ≤ 0.2) were prepared by aqueous nitrate–citrate sol–gel synthesis route using citric acid (CA) as a complexing agent. Lanthanum (III) and europium (III) nitrates La(NO3)3 (99.0%, High Purity Chemicals), Eu(NO3)3 (99.0%, High Purity Chemicals), calcium (II) nitrate Ca(NO3)2 (99.0%, High Purity Chemicals), and ammonium metavanadate NH4VO3 (99.0%, High Purity Chemicals) were used as starting compounds. They were weighted according to the desired stoichiometric molar ratio. Nitric acid (HNO3), distilled water, and ammonia (NH3·4H2O) were used as solvents and reagents to regulate the pH of the solutions. Firstly, NH4VO3 was dissolved in concentrated ammonia solution by stirring at 70–80 °C temperature. Then, the CA dissolved in distilled water with a small amount of NH3·4H2O was added. Next, La(NO3)3, Eu(NO3)3, and Ca(NO3)2 were added. To prevent precipitation, the pH that reached the value of ~6–7 was controlled. Finally, the same amount of the aqueous solution of the complexing agent CA was repeatedly added to the reaction mixture to prevent crystallization of metal salts during the gelation process. The clear solution was concentrated by slow evaporation at 80–90 °C in an open beaker. A transparent gel has been formed after evaporation of nearly 90% of the water during the continuous stirring. The powders were obtained after drying in an oven at 100 °C. The powders were step by step calcined for 5 h at the range from 150 to 700 °C. Figure 1 illustrates the process of synthesis.
Fig. 1

Schematic diagram showing the experimental process for synthesis of the La1 − x Ca x VO4 and La1 − x − y Eu y Ca x VO4 (0 ≤ x, y ≤ 0.2) compounds

Equipment

Phase compositions and crystal lattice parameters of the synthesized samples were determined using X-ray diffractometer DRON-3M (CuKα-radiation with a Ni filter). The diffraction patterns were taken with a 2°/min step. Characterization of the samples’ morphology was made using scanning electron microscope (SEM) Tescan Mira 3 LMU with 20-nm electronic beam diameter. The secondary electron detector (InBeam) could be used, if there is a need, to enhance spatial resolution up to 1 nm. Infrared (IR) spectra of the samples were recorded on PerkinElmer IR spectrometer using the KBr pellet method in the 1400–400 cm−1 range. Luminescence spectra were measured at high resolution equipment using DFS-12 and DMR-23 diffraction spectrometers with excitation by diode lasers, nitrogen gas laser, and xenon lamp (see more details in [26, 27]).

Results and Discussion

X-ray Diffraction

The X-ray diffraction (XRD) patterns of the synthesized La1 − x Ca  VO4 and La1 − x − y Eu y Ca x VO4 (x ≤ 0.2, y ≤ 0.2) samples are shown in Fig. 2. It was found that the La0.9Ca0.1VO4 sample is crystallized in monoclinic structure; a space group is P21/n (see Fig. 2). This result is well matched with a standard card of monoclinic LaVO4 (JCPDS PDF2 50-0367) (see Fig. 2). The La0.9Eu0.05Ca0.05VO4 sample with low concentration of the Ca2+ and Eu3+ impurities is also attributed to the monoclinic structure. Proximity of the Ca2+ (1.12 nm), La3+ (1.16 nm), and Eu3+ (1.066 nm) ions radii provides an opportunity for calcium and europium ions’ incorporation into three plus cation’s sites. Besides, we see that small quantities of the Ca2+ (x ≤ 0.1) and Eu3+ ions (y ≤ 0.05) can enter to the crystal lattice without change of the monoclinic LaVO4 structure.
Fig. 2

X-ray diffraction patterns of the La1 − x Ca x VO4 and La1 − x − y Eu y Ca x VO4 (0 ≤ x, y 0.2) samples

Increasing concentrations of the Eu3+ and Ca2+ impurities leads to formation of the monoclinic and tetragonal phase mixture (Fig. 2). The La0.6Eu0.2Ca0.2VO4 sample becomes related to tetragonal structure, belonging to the I41/amd space group. This result is agreed with the standard card of tetragonal LaVO4 (JCPDS PDF2 32-0504) (see Fig. 2). If some amount of the residual monoclinic phase is still presented in these samples, it was below the detection limit of our XRD equipment.

Therefore, increasing concentration of europium and calcium ions in La1 − x − y Eu y Ca x VO4 (x, y ≤ 0.2) solid solutions leads to the change of the crystal structure, and subsequently, stabilization of the tetragonal phase takes place.

Morphology and Chemical Element Analysis

The set of characteristic SEM images of the La1 − x − y Eu x Ca y VO4 samples are shown in Fig. 3. The top two images, a and b, show areas of ~5 × 5 μm; in other words, general view of samples is given, whereas other images, c–f, show small separated areas, up to ~0.5 × 0.5 μm size. It is easy to see from general view that samples consist of grains of different sizes, from tens of nanometers to 1 μm. Grains of large sizes (0.2–1.0 μm) are agglomerates of smaller ones, and they have no certain shapes. Detailed view of agglomerates and of small grains (Fig. 3c–f) show that they are formed by nanoparticles of size from 20 to 100 nm. The La0.9Eu0.05Ca0.05VO4 samples with low concentration of the Eu and Ca impurities consist of grains of 0.1–0.2 μm-size, those are formed by nanoparticles of 10–20-nm size (Fig. 3c). The nanoparticles of larger size, ~100 nm, are characteristic for the samples with higher concentration of dopants, La0.8Eu0.1Ca0.1VO4. Most of them have polyhedral shapes with clearly defined edges and angles between them (Fig. 3d). The particles with the observed polyhedral shapes have to be assigned to the crystal system of lower symmetry. Thus, most likely, they should be regarded as a manifestation of the monoclinic phase of the studied samples. The next increase of the Eu3+ and Ca2+ dopant concentrations also generates particles of ~100-nm sizes (Fig. 3e). Despite of the poor quality of this image, it is possible to conclude that the shapes of the particles are closer to the cylinders and rods.The same shapes are observed for the samples of the highest dopant concentration, La0.6Eu0.2Ca0.2VO4 (Fig. 3f). However, it should be noted that size of the particles is smaller for these samples; it is preferably 30–60 nm.
Fig. 3

The SEM images of the La0.8Ca0.2VO4 (a), La0.6 Eu0.2Ca0.2VO4 (b, f), La0.9 Eu0.05Ca0.05VO4 (c), La0.8 Eu0.1Ca0.1VO4 (d), and La0.7 Eu0.15Ca0.15VO4 (e) samples

Thus, we can say that the change of the powder samples’ composition is accompanied by the changes in the size and morphology of their constituent particles. These changes are consistent with the XRD data on the structure and phase composition of the samples. Therefore, the SEM data can be interpreted as indicating that the samples of higher dopant concentration are mixtures of monoclinic and tetragonal phases and the content of the latter increases when the content of europium and calcium ions increases.

Chemical element analysis was performed for 3–5 various agglomerates or grains for each sample using SEM tools. Monitoring area from which information about content of atoms have been taken was larger than e-beam size, and it was near (30–40) × (30–40) μm. We have found that La, Eu, Ca, V, and O are the main components of the samples. In certain cases, some quantity of carbon atoms revealed that obvious occurred when edge zone of grains or grains of small thickness were monitored. (Powder samples were fixed on special carbon-covered scotch tape for SEM measurements.) Average data are accumulated in Table 1.
Table 1

Calculated starting composition of the La, Ca, and Eu atoms in the samples under synthesis, average content of atoms (in at.%), and evaluated composition of the synthesized samples

Starting composition

La

Eu

Ca

Evaluated composition

La0.8Ca0.2

41.2

9.4

La0.81Ca0.19

La0.9Eu0.05Ca0.05

26.60

0.73

1.76

La0.92Eu0.02Ca0.06

La0.8Eu0.10Ca0.10

20.81

2.34

1.11

La0.86Eu0.10Ca0.04

La0.7Eu0.15Ca0.15

12.30

3.30

1.73

La0.71Eu0.19Ca0.10

La0.6Eu0.20Ca0.20

12.88

7.07

2.97

La0.56Eu0.31Ca0.13

Analysis of data presented in Table 1 gives possibility to note the next.
  1. 1.

    Data of the SEM experiments on the contents of the La3+ ions in the synthesized samples (first column of the Table 1) are quite closed to the expected compositions (see the far right column of the Table 1).

     
  2. 2.

    If we take the total content of the Eu3+ and Ca2+ ions, it is easy to see that the experimentally measured data are quite close to the expected compositions. However, it is easy to see that the predicted simultaneous fourfold increase of ions Eu3+ and Ca2+ content (from 0.05 up to 0.20 at.%) is not implemented for the actual synthesized samples. We can observe when the content of the Eu3+ ions increases by ~10 times, the content of the Ca2+ ions increases only by ~2–3 times. This is due both to specific of the samples (heterogeneity of sizes, shapes, and thickness of grains of powder, etc.) and features of the method (a significant error for light elements, the effect of electron beam on oxide surfaces, etc.). Nevertheless, this result should be taken into account when describing other properties of the synthesized compounds. First of all, it concerns their luminescent properties. To avoid misunderstanding, we will not change following designation of samples noted in the “Synthesis” and “X-ray Diffraction” sections.

     

IR Spectroscopy

IR spectroscopy study of the La1 − x Ca x VO4 and La1 − x − y Eu y Ca x VO4 (0 ≤ x, y ≤ 0.2) samples was performed to confirm their structure and composition. The view of the IR spectra in the range of 400–1100 cm−1 (Fig. 4) is typical for the LaVO4 IR absorption spectra previously measured in this range [2830]. These spectra are also very similar to the IR spectra of other lanthanide orthovanadates LnVO4 (Ln = Y, Ce−Yb) [28, 31], multi-metal orthovanadates such as M3Ln(XO4)2 and M2MLn(XO4)2 (M, M = Na, K, Rb) [32] and vanadates of the M′′ 3V2O8 (M′′ = Mg, Ca, Sr, Ba, and Zn) type [3335]. Namely, the spectra are similar to all of compounds; those lattices are built by isolated tetrahedral VO4 3− molecular groups.
Fig. 4

IR absorption spectra of the LaVO4 (1), La0.9Ca0.1VO4 (2), La0.8Ca0.2VO4 (3), La0.8Eu0.1Ca0.1VO4 (4), and La0.6Eu0.2Ca0.2VO4 (5)

It is well known that the bending (ν2 and ν4) and stretching vibrations (ν1 and ν3) of O–V–O bonds of the VO4 3− anion form IR absorption spectra of various orthovanadates in the range 400–700 and 700–1100 cm−1, respectively [36, 37]. In fact, measured by us, spectrum of the undoped LaVO4 contains separated weak bands located in the range 400–700 cm−1, while stronger wide band lies in the range 700–1100 cm−1 (Fig. 4, curve 1). Sharp peak at 434 cm−1 and blurred band centered near 550 cm−1 compose IR spectra of this sample in the former range. The main peak at ~778 cm−1 and three strongly overlapped other ones located at ~820, 850, and 880 cm−1 form wide band (Fig. 4, curve 1). (All the mentioned peaks are marked by arrows close to the curve 1 in Fig. 4, and their positions are in Table 2).
Table 2

IR peaks positions (in cm−1) and their attributions to vibration modes in the VO4 3− vanadate groups

LaVO4

La0.9Ca0.1VO4

La0.8Ca0.2VO4

La0.8Eu0.1Ca0.1VO4

La0.6Eu0.2Ca0.2VO4

Vibration modes

434

438

436

439

440

ν4

 

570

553

553

 

778

780

777

783

780

 

802

801

802

801

 
   

811

ν3

820

822

824

821

820

 

850

836; 850

835; 850

835; 854

836; 852

 

880

874

875

875

874

ν1

1026

976

975

918; 978

975

 

When LaVO4 is doped by the Ca2+ ions, the shapes of spectral bands and peaks positions are changed, but these changes are not as striking as one could expect, but we should note that composition of peaks changes. As a result, six peaks marked by arrows can be found on the curves 2 and 3 of Fig. 4 in the range of wide absorption band.

If the Eu3+ ions are added to the La1 − x Ca x VO4 compounds, then band of stretching vibrations becomes more complicated and its components are more overlapped (Fig. 4, curves 4 and 5). Thus, six or seven peaks can be distinguished there (see arrows), and shape and structure of the band become very similar to ones previously published on IR absorption spectra of the LaVO4 containing Eu3+ ions [31, 36]. (The peaks positions of all measured IR absorption bands are accumulated in Table 2.)

The exact assignment of spectral components to a certain type of modes is difficult, especially that concerns stretching vibration range. Due to the low Cs symmetry of the VO4 3− molecular anion in monoclinic LaVO4, four IR lines lying close to each other can appear in this range of spectra. Our experimental findings are in good agreement with noted above theoretical prediction. Mentioned fact causes strong overlapping of the lines, that is why they are revealed as one complex wide band. Doping with calcium ions causes a distortion of certain amount of vanadates groups. As a result, additional peaks appear in the spectrum. Besides, as we have shown above, all co-doped samples under our study are a mixture of monoclinic and tetragonal LaVO4 phases and contribution of the phases depends on the dopant concentration. The VO4 3− molecular groups possess D2d symmetry in tetragonal LaVO4. So, two lines of Eu and A2u symmetry can be found in the range of stretching vibrations if morphology of the VO4 3− groups corresponds to ideal tetragonal lattice structure. Really, neighbor environment and symmetry of some VO4 3− groups in tetragonal lattice are also deformed by Ca2+ ions, and all of VO4 3− internal vibrations can occur in the IR spectra. As previously have been reported, the calculated [38] and measured [31] peak positions of the IR absorption lines for tetragonal LaVO4 are close to those for monoclinic LaVO4. Thus, the eight lines can form the range of stretching vibrations, we suppose. Change of the phase composition leads to mentioned changes of the shape of the IR bands and their positions. Regarding published theoretical and experimental data, a possible assignment of all measured features was made (Table 2). Additional studies are necessary in order to clarify the origin of the bands that are in the ranges 500–700 and 900–1000 cm−1. We can only note that similar features were previously observed in the IR absorption spectra of some Ca containing orthovanadates [33, 34].

In any cases, we are able to state that observed IR spectra confirm that anionic sub-lattice of studied vanadates is built by VO4 3− molecular anions.

Luminescent Spectroscopy

Emission of the La1 − x Ca x VO4 samples is observed in a wide spectral range from 400 to 800 nm. The observed bands are complex. The photoluminescence (PL) spectra contain two wide and strongly overlapped components at position of their maximum near ~570 and ~630 nm (Fig. 5). The band positions and structure of the spectra are similar to those described previously as for undoped nanosized LaVO4 and Ba3V2O8 compounds, as well as for activated Sr2.91V2O8:0.06Eu3+, Ca2NaMg2V3O12:Eu3+, and Na2LnMg2V3O12 vanadate phosphors [17, 21, 39, 40]. The observed bands can be assigned to electron transitions in the VO4 3− groups; those are constituents of mentioned compounds. The charge transfer transitions of 3d orbitals electron of the V5+ ion to the 2p orbital electron of the O2− ion inside of the VO4 3− groups are the nature of this luminescence [40, 41]. According to energy scheme of the vanadate tetrahedron, emission bands at 570 and 630 nm are connected respectively with 3T2 and 3T1 → 1A1 electron transitions in the VO4 3− groups [3941]. These transitions are forbidden in the ideal Td symmetry of the VO4 3− group. Distortion of the VO4 3− tetrahedrons from the ideal symmetry in the crystal lattice of LaVO4 enhances the spin–orbit interaction and makes these transitions partially allowed [42]. As we noted above, in the monoclinic phase of the La1 − x Ca x VO4 samples, the VO4 3− groups have distorted symmetry. Besides, some of these groups are affected by Ca2+ ions. The VO4 3− group also is of lower symmetry than Td in tetragonal La1 − x − y Eu y Ca x VO4 samples. Thus, we have all the reasons to assign the observed 570 and 630 nm bands to the 3T2 and 3T1 → 1A1 electron transitions in the VO4 3− groups.
Fig. 5

Emission spectra of the La1 − x Ca x VO4 samples, λex = 310 (13) and 290 nm (4); x = 0 (1), 0.2 (2), and 0.1 (3, 4), T = 77 K

Emission of the La1 − x − y Eu y Ca x VO4 samples is observed in the 550–730 nm spectral range and consists of narrow spectral lines, and undoubtedly, they are caused by radiation transitions in Eu3+ ions (Figs. 6 and 7). The emission intensity increases if content of dopants increases in the range (0 ≤ x ≤ 0.15) (Fig. 7).
Fig. 6

Emission spectra of the La0.8Eu0.1Ca0.1VO4 (14) and La0.9Eu0.1VO4 (5) samples at λex = 532 (1, 5), 473 (2), 405 (3), and 337 nm (4)

Fig. 7

Emission spectra of the La1 − x − y Eu y CaxVO4 (14) and La1 − x Eu x VO4 (5) samples at λex = 405 nm; x, y = 0.05 (1), 0.1 (2, 5), 0.15 (3), 0.2 (4)

Excitation spectra of all the synthesized samples are typical for luminescence excitation spectra of undoped and doped with Eu3+ ions LaVO4 [2023, 4244]. They contain wide band with peak position at about 320 nm (Fig. 8). Also, two narrow weak peaks at ~405 and ~475 nm are presented in the spectra of the Eu-containing samples.
Fig. 8

Excitation spectra of the LaVO4 (1), La0.9Eu0.1VO4 (2), and La0.6Eu0.2Ca0.2VO4 (3, 4) samples at λem = 550 (1), 614 (2, 3), and 622 nm (4); T = 77 K (1), T = RT (24)

Structure of the emission spectra depends on the excitation wave length, λex (Fig. 6), and on the concentration of the Ca2+ and Eu3+ dopants (Fig. 7). For the La0.8Eu0.1Ca0.1VO4 sample, which is just a mixture of monoclinic and tetragonal phases (see Fig. 2), the PL lines at 586, 592.5, 607, 611, 614, 618, 649, 697, and 702 nm (marked by arrows at Fig. 6, curve 2) are observed at all applied λex. (We call this set of the lines as first set.) Note also that the ratio of intensity of the lines at 611, 614, and 618 nm changes if λex varies. Besides, intensive lines at 580, 590, 591.7, 595.6, 608.7, and 622 and lines near 645, 651, 655, 706, and 708 nm (marked by arrows with asterisk at Fig. 6, curve 3, and at Fig. 7, curve 4) are distinctively observed in the spectra measured at λex = 405 nm. (We call this set of the lines as second set.) So, the measured emission spectra are the superposition of two sets of luminescence lines. Since we have shown above that the sample La0.8Eu0.1Ca0.1VO4 is just a mixture of two crystalline phases, we have first to suppose that these sets are related with emission of Eu3+ centers in monoclinic and tetragonal phases.

With the intent to clarify this assumption, we performed study of the luminescence spectra dependence on the samples composition at the same excitation wave length, λex = 405 nm (Fig. 7). We found that the emission spectra of the samples where x = 0.1 and 0.15 are very close to each other (Fig. 7, curves 3 and 4). This finding agrees with the statement that these samples are the mixture of two phases. Tetragonal phase dominates in the La0.6Eu0.2Ca0.2VO4 sample, and we see that second set of the luminescence lines also dominates in the spectrum of this sample, but we see also that mentioned before line at 618 nm is vanished in this spectrum (Fig. 7, curve 4). When observe the emission spectrum of the La0.9Eu0.05Ca0.05VO4 (Fig. 7, curve 1), we see that first set of lines mainly reveals in the spectrum (Fig. 7, curve 1). As the La0.9Eu0.05Ca0.05VO4 sample is monoclinic (see Fig. 2), we should relate the first set of lines to emission of Eu3+ ions in monoclinic crystal phase. At the same time, we note that line at 618 nm is also vanished in this spectrum. So, we concluded that not only competition between content of the crystal phases influence luminescence behavior. Let us discuss the question in detail.

The observed narrow PL lines are caused by the 5D0 → 7FJ (J = 0, 1, 2, 3, 4) electron radiation transitions in the inner 4f n shell of the Eu3+ ions. The 7FJ energy levels may be split in the crystal field on the 2j + 1 sub-levels, but some of possible transitions can be forbidden by the symmetry rules [45]. If the Eu3+ ions occupy the site of the La3+ ions in tetragonal LaVO4 crystal lattice (I41/amd space group), they have D2d site symmetry. In such a case, group theory predicts no luminescence peaks from 5D0 → 7F0 transition, two lines from the 5D0 → 7F1 and three lines from the 5D0 → 7F2 transitions. If the Eu3+ ions occupy the site of the La3+ ions in monoclinic LaVO4 crystal lattice (P21/n space group), they have C1 site symmetry. In this case, group theory predicts manifestation all of possible transitions: one luminescence peak from the 5D0 → 7F0 transition, three lines from 5D0 → 7F1 and five lines from the 5D0 → 7F2 transitions [44].

So, it was not surprising that accounting the spectra in Fig. 6 in sum, we have found one line for the 5D0 → 7F0 (spectral range 570–585 nm), five lines (586, 592.5, 590, 591.7, 595.6 nm) for the 5D0 → 7F1 (spectral range 585–600 nm), and seven lines (607, 614, 618, 608.7, 611, 614, 622 nm) for 5D0 → 7F2 transitions (spectral range 600–650 nm), as this figure shows the luminescence spectra of the sample which is mixture of the monoclinic and tetragonal phases. This statement also concerns the spectra 2 and 3 in Fig. 7, as they also represent luminescence of the mixture, x = 0.1 and 0.15, respectively. It was surprising that we see intensive line from the 5D0 → 7F0 transition for the La0.6Eu0.2Ca0.2VO4 sample which represents only tetragonal crystal structure (Fig. 7, curve 4). Moreover, we see that the spectrum of the monoclinic La0.9Eu0.05Ca0.05VO4 sample (Fig. 7, curve 1) is lesser complicated than it might be expected to this structure. At the same time, it is important that luminescence intensity of these samples is lower if compared to intensity for other samples shown in this Figure. As we go from spectrum 1 to spectrum 4 in Fig. 7, the contribution of monoclinic phase decreases. These results mean that other factors, not only crystal structure, significantly determine luminescence behavior of the samples under study. Thus, we supposed that as shown in Figs. 6 and 7, curves 2–4, spectral transformations are mainly related to luminescence behavior of tetragonal phase, especially to role of Ca2+ cations, as their concentration increases by 2–3 times when gone from x = 0.1 to 0.2 (see the “Morphology and Chemical Element analysis” section).

In fact, obtained results mean that at least two of different type luminescence centers formed by the ions Eu3+ in the sample of tetragonal structure contribute to the PL spectra. We ascribed the abovementioned spectral lines to emission of type I (“first set” of lines) and type II of the centers (“second set” of lines). Emission line from the 5D0 → 7F0 transition (580 nm) is observed only for the type II centers, and the most intensive lines of the 5D0 → 7F2 and 5D0 → 7F4 transitions for these centers are located at 622 and 708 nm, respectively. For type I centers, the most intensive lines are located at 614 and 697 nm. These spectral features allow us to do assumption that types I and II of centers are characterized by different symmetry of the Eu3+ ions environment: the type I centers are characterized by higher site symmetry and the type II centers are characterized by lower site symmetry in the crystal lattices [45]. We supposed that type I centers are formed by the Eu3+ ions in their regular positions in the crystal lattice (D2d symmetry).

As for the type II centers, we suppose that they can be formed by Eu3+ ions disturbed by defects generated as a result of Ca2+ cations incorporation into LaVO4 crystal lattice. To confirm this supposition, we performed additive experimental study of the sample that does not contain Ca2+ ions, namely, La0.9Eu0.1VO4 composition of tetragonal structure [14, 23] (Figs. 6 and 7, curves 5). Taking into account that above noted lines of type II centers (e.g., lines at 622 and 708 nm) were not observed for the Ca2+-free sample (Fig. 7, curve 5), we concluded that the most possible origin of the type II centers is the Eu3+ ions disturbed by Ca-induced defects. In the case of the doped with calcium LaVO4 lattice, the Ca2+ dopant has to replace the La3+ ions. Then, effective “−1” charge arises. Charge compensation of mentioned −1 charge is needed, as the crystal has to be electro-neutral. This compensation can be achieved via formation of one oxygen vacancy, [VO]2+, on each two Ca2+/La3+ replacements. Thus, for the Ca2+ dopants arrangement in the LaVO4 crystal lattice, we should expect that some of the Eu3+ ions are under the effect of both neighbor Ca2+ cations and oxygen vacancy. Thus, we can assume that such Eu3+ ions are just the type II of the luminescence centers, and their local symmetry is lower if comparing to symmetry of the type I centers. Taking into account that 5D0 → 7F0 transition emission line (near 580 nm) is observed in the spectra (Figs. 6 and 7), the symmetry of type II centers may be C2 or Cs [44, 45]. The C2 symmetry of the Eu3+ surrounding in the I41/amd space group of orthovanadates can be achieved only by equal distortions of two oxygen positions. Therefore, we assume that case of the Cs symmetry of the Eu3+ surrounding at the type II centers is more probable.

Excitation spectra measured in the emission lines corresponded to different types of centers (614 and 622 nm for types I and II centers, respectively) confirmed made above description, as these spectra revealed some differences (Fig. 8). The wide band at 320 nm is more intensive in the excitation spectra of type I centers. The 320-nm band is also observed in the spectra of the La0.9Eu0.1VO4 sample (Fig. 8, curve 2). This band is caused by electron transitions in the vanadate VO4 3− groups. Therefore, the type I centers are better excited through the matrix than type II centers, that confirms our assumptions about relation of the type I centers with the Eu3+ ions in the regular positions in the crystal lattice.

Total emission intensity of the La1 − x − y Eu y Ca x VO4 samples have shown essential increasing when dopant concentration increases up to x, y = 0.15 (Fig. 9, curve 1). Similar dependence reveal peak intensity of the 614 and 622 nm lines caused by luminescence of types I and II centers, respectively (Fig. 9, curve 2 and 3), but we see that main contribution to luminescence enhance is related with emission of type II of centers. Therefore, spectral behavior of the type II luminescence centers is not only single manifestation of the Ca2+ cations influence on the La1 − x − y Eu y Ca x VO4 emission. The Ca2+ doping enhances also efficiency of this type of luminescence centers.
Fig. 9

Dependencies of the La1 − x − y Eu y Ca x VO4 luminescence intensity on dopant concentration x; total emission (1) and 622 nm peak intensity (2, 3); λex = 405 (1, 2) and 473 nm (3)

As for quenching of the luminescence intensity at x, y > 0.15, it should be noted that several mechanisms may be responsible for this. First, it is concentration quenching related with Eu3+ content. Then, as we described in the “Morphology and Chemical Element Analysis” section, the sizes of particle decreases for the La0.6Eu0.2Ca0.2VO4 sample. So, the surface quenching can be important mechanism in this case.

Surely, made assumption about role of the Ca2+ ions in luminescence of the lanthanum vanadate, LaVO4, co-doped with Eu3+ and Ca2+ ions requires additional study, and we plan to carry out it in the future.

Conclusions

The La1 − x Ca x VO4 and La1 − x − y Eu y Ca x VO4 (0 ≤ x, y ≤ 0.2) micro/nanosized powders were prepared by aqueous nitrate–citrate sol–gel synthesis. Phase composition of the sample depends on the x and y values.

The La0.9Ca0.1VO4 is crystallized in monoclinic structure up to the x = 0.1.

The La0.9Eu0.05Ca0.05VO4 sample was also attributed to the monoclinic structure.

Increasing concentration of europium, Eu3+, and calcium ions, Ca2+, in La1 − x − y Eu y Ca x VO4 solid solutions leads to the change of the crystal structure, and subsequently, stabilization of the tetragonal phase takes place.

Phase transformation and especially Ca2+ ions influence IR spectroscopy and luminescence behavior of studied compounds, as Ca2+ ions impact both on VO4 3− molecular groups and La3+ and Eu3+ ions.

Emission of the La1 − x Ca x VO4 samples consists of wide bands in the 450–800 nm spectral range. These bands were ascribed to electron transitions in the distorted VO4 3− vanadate groups.

Emission of the La1 − x − y Eu y Ca x VO4 samples is observed in the 550–730 nm spectral range, and it consists of narrow spectral lines. These lines are caused by the 5D0 → 7FJ electron transitions in the Eu3+ ions. The Eu3+ ions form two types of emission centers in the samples under study. The assumption was made that type I centers are formed by the Eu3+ ions in their regular positions in the crystal lattice, while the type II centers have complex structure and they consists of Eu3+ ions, Ca2+ cations, and oxygen vacancies.

Abbreviations

IR: 

Infrared

PL: 

Photoluminescence

RE: 

Rare earth

SEM: 

Scanning electron microscopy

XRD: 

X-ray diffraction

Declarations

Acknowledgements

The authors thank the leaders and people of the “NanoMedTech LLC” company (Kyiv, Ukraine), who provided modern SEM equipment for the study samples, for their help during the study. Authors are grateful to Leading Engineer Iryna Nedielko (Taras Shevchenko National University of Kyiv) for her help in the XRD measurements.

Funding

Publication is based on the research provided by the grant support of the State Fund for Fundamental Research (project F64/44-2016).

Authors’ Contributions

SN contributed to the development of strategy of the work and its scientific management, luminescent SEM study, and processing and discussion of the measurement results. OCh contributed to the luminescent spectra measurements, processing and analysis of taken results, and discussion of luminescence and IR spectroscopy results. SAN contributed to the development of strategy of the chemical part of this work and its scientific management, synthesis adjustment, and analysis of IR spectroscopy and XRD results. AS contributed to the solution for synthesis and samples’ preparation, IR spectroscopy measurement and processing of measurement results, XRD study, and processing and analysis of their results. TV contributed to the study, analysis, and discussion of the synthesis results, IR spectroscopy and XRD study, and discussion of the measurement results. All authors read and approved the final manuscript.

Competing Interests

The authors declare that they have no competing interests.

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Authors’ Affiliations

(1)
Faculty of Physics, Taras Shevchenko National University of Kyiv
(2)
Faculty of Chemistry, Taras Shevchenko National University of Kyiv

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Copyright

© The Author(s). 2017